1. Field of the Invention
The present invention is directed to apparatus for thermal spray coating, and particularly to a portable thermal spray coating gun for applying a polymer-containing coating material to a substrate.
2. Background of the Art
The term “thermal spraying” refers to process in which a coating material feedstock is heated and propelled as individual droplets or particles onto the surface of a substrate. The coating material is heated by the applicator (e.g., a thermal spray gun) by using combustible gas or an electric arc and converted into molten or plastic droplets or particles, which are propelled out of the spray gun by compressed gas. When the coating material particles strike the substrate they flatten and form thin platelets (“splats”) that adhere to the surface of the substrate. The splats cool and build up a layer of applied coating material having a laminar structure.
Various types of thermal spray guns are known. For example, U.S. Pat. No. 5,285,967 to Weidman discloses a high velocity oxygen-fuel (“HVOF”) thermal spray gun for spraying a melted powder composition of, for example, thermoplastic compounds, thermoplastic/metallic composites, or thermoplastic/ceramic composites onto a substrate to form a coating thereon. The gun includes an HVOF flame generator for providing an HVOF gas stream to a fluid cooled nozzle. A portion of the gas stream is diverted for preheating the powder, with the preheated powder being injected into the main gas stream at a downstream location within the nozzle. Forced air and vacuum sources are provided in a shroud circumscribing the nozzle for cooling the melted powder in flight before deposition onto the substrate.
Thermal spray guns typically use mixtures of oxygen-fuel gas, air-fuel gas, air-liquid fuel, oxygen-liquid fuel, or electric arc, and plasma as a heat medium to melt and propel the individual droplets to a prepared substrate. Thermal spray devices fall within general classification of equipment: (1) wire combustion, (2) powder combustion, (3) twin wire electric arc, (4) plasma-powder, (5) high velocity oxygen-fuel gas-powder, (6) high velocity oxygen-fuel gas-wire, (7) high velocity air-liquid fuel-powder, (8) high velocity oxygen-liquid fuel-powder, (9) detonation gun powder, and (10) water cannon plasma. In general thermal spray devices are wire combustion, powder combustion, plasma and electric arc.
In the wire combustion process a combustion heat source is initiated and feed stock material in wire or rod form is driven into the heat medium where a compressed air stream concentrates the heat source about the axially fed feed stock whereby it is melted atomized and propelled to the substrate for deposition of the coating.
Attempts have been made to spray polymer materials in wire form using existing wire combustion technology; however, they have not succeeded as the air compression wave required to atomize the polymer wire is oriented so as to impinge the high temperature flame directly onto the feedstock material and thereby consuming the resultant atomized droplets. The high temperature associated with this device can cause embrittlement of the coating. The existing wire combustion technology uses a siphon plug to mix the oxygen and fuel gas prior to combustion. This is a complicated and expensive component to machine.
In the powder combustion process a combustion heat source is initiated and feed stock material in powder form is introduced axially or tangentially to the propagated flame. The feedstock powder material is delivered by means of a powder feeder or gun mounted hopper.
The powder combustion process has been used to apply polymer materials; however, the flame temperature consumes 50 percent or more of the feedstock material. Additionally, the relatively high temperature can burn the subsequent applied coating and/or cause embrittlement of the coating. The existing powder combustion technology uses a siphon plug to mix the oxygen and fuel gas prior to combustion. This is a complicated and expensive component to machine. Combustion powder equipment does not provide for the generation of an aligned and oriented compression wave nor does it provide for cooling mixture air in the nozzle body whereby the flame temperature can be lowered.
In the electric arc process two feed stock wires of similar or dissimilar material with opposite polarity are fed into a spray device where they are directed to impinge one upon the other and thereby strike an arc producing rapid melting of the feed stock materials. A concentrated compressed air stream atomizes the molten material and propels it to a substrate. The generating source for the electric arc is a MIG welding rectifier where the positive charge is applied to one feedstock material wire and the negative or ground is applied to the other feedstock material wire.
The electric arc requires material in wire form which must be electrically conductive and therefore is not suitable as a means of spraying plastic materials.
In the plasma powder system a heat source is generate by passing an inert gas between the gap formed by an electrode and nozzle which are at an electrical potential. A high voltage, high frequency, low amperage arc is struck which bridges the gap between the electrode and nozzle. This small amperage arc partially ionizes the inert gas and generates a conductive path for the low voltage, high amperage potential to complete a circuit. The inert gas is thereby totally disassociated expands and exits the nozzle bore at high velocity. During the recombination of the disassociated gas heat is generated which is used to melt the feedstock material powder injected into the plasma flame tangentially. The velocity of the flame propels the feedstock material powder onto a substrate.
The plasma gun has been used to spray high temperature polyester with an aluminum constituent component but the intent is to burn off some of the polymer material. The operating cost of the equipment further limit it as a device for economical on-site application of powder paint materials.
In the detonation gun system a heat source in propagated by a series of controlled explosions. An oxygen-fuel gas mixture is injected into a chamber by a means similar to the valve in an internal combustion engine. However, the chamber is open at one end and there is no piston. The oxygen-fuel gas mixture is ignited by a spark plug which is coordinated with the valve train. The fuel and ignition cycle is repeated multiple times per second and the resultant detonation wave melts and propels the feedstock material to a substrate. The feedstock material is delivered in powder form from a powder feeder device.
The detonation gun is large and requires a dedicated room. It cannot be used on site. It is used to apply hard dense coatings and is not suitable for polymer materials.
High velocity is unsuitable for applying polymer materials in that the pressures required for the fuel and oxidizing medium gases ensure a large flame and high temperature. Also, the very high velocity is detrimental to the plastic droplets. The temperature of the flame can degrade and embrittle the applied coating. Further, the high operating cost of the equipment precludes it from ever becoming a viable means of applying low cost polymer materials.
Powder feeders come in a variety of constructs; but, the basic function is to convey material in powder form. These constructs are fluidized bed with venturi delivery, mechanical screw with venturi delivery, gravity fed with venturi delivery, meter wheel with venturi delivery. Powder feeders are required to deliver feedstock materials in powder form, to various equipments, from a material source which, is detached from the said equipment. This equipment can be a thermal spray device, electrostatic powder paint gun, extrusion screw and injection molding equipment. In all cases a feeder which delivers precisely metered and non pulsed material is essential. This is particularly true for thermal spray powder combustion equipment and electrostatic spray guns.
Current fluid bed venturi powder feeder technology is insufficient for use in thermal spray devices and electrostatic powder paint guns. In both electrostatic and thermal spray equipment the pressure, velocity and flow required at the nozzle to deliver the feedstock material to the substrate, is different than the pressure, velocity and flow required to generate a vacuum and meter feedstock material (spray rate). Currently used equipment uses the same pressure, velocity and flow source for both meter and delivery functions. This is a compromise of two separate functions. The mechanical screw/venturi and the meter wheel venturi separate the functions but they are subject to binding, wear, and pulsing from uneven feed into the wheel or screw.
Powder paint equipment delivers polymer/powder paint materials to a substrate via an electrostatic spray gun. This gun applies an electrical charge to the feedstock material which is at a different charge to the substrate to be coated. The coated part is placed in an oven whereby the electrically attached polymer materials are melted and cured. In a second embodiment of this technology the substrate to be coated is placed in an oven and heated above the melting point of the polymer material to be applied. The heated part is then dipped into a fluidized bed of the feedstock polymer powder whereby the material in contact with the heated part melts and is deposited onto the substrate.
Both embodiments have limitations to their use. They require high energy cost to operate the oven. They cannot be used on site as they are factory fixed facilities. The parts that can be coated are limited by the size of the oven available. In the case of the electrostatic equipment certain combinations of metals and or conductive polymers may be precluded as it can affect the charge.
As stated previously, existing thermal spray technology has been used in an attempt to apply thermally sprayed polymer materials in powder form with very limited success. Additionally, equipment has been produced that is dedicated to the application of polymer powder materials. The heat sources for these apparatus are oxygen-fuel gas or propane-air. They function like typical thermal spray powder combustion guns. However, they address the temperature requirements of polymer material somewhat better than higher temperature thermal spray combustion powder apparatus designed for metal and ceramic materials.
There are limitations to the effectiveness of these apparatus. They do not address the separate function requirements of particle velocity and flow in the heat medium and the pressure and flow required to supply a measured spray rate consistent with the thermal output of the gun. They either supply the correct spray rate for the material utilized and the thermal output or they provide a correct velocity and flow to effect an appropriate dwell time or they compromise on both. Furthermore, all prior embodiments of these apparatus use a siphon plug gas mixing device. In the case of the propane-air heat source the function of the stoichiometry of the flame is not separated from the air used to provide for the correct velocity and flow of the feed stock materials. As additional air is introduced into the flame to propel the particle the temperature of the flame is raised as the proper mix of propane and oxygen is attained. In all prior known embodiments of the apparatus the temperature of the flame is too high as they do not address the requirement of cooling the flame before it contacts the polymer feedstock materials. This high temperature results in polymer feedstock materials combusting or acting as an additional fuel source when in contact with the flame. It is indicated by the bright orange flame generated when polymer materials are introduced into the heat medium. This combustion of the feedstock materials results in reduced deposition rates below 50%. Additionally, it precludes the use of electrostatic grade (the 5 to 160 micron range) materials which provide for a more homogenous and smoother coating. This limits the equipment generally to fluidized bed materials which, are in the 80 to 200 micron range and deliver rougher coatings. As the temperature of existing apparatus is too high they do not address the need for a compression wave to effect efficient transfer of a reduced temperature heat source to the polymer material feedstock. The prior embodiments rely on preheating the substrate to 400° F. prior to application of the polymer feedstock material to achieve a viable deposited coating. All technology up to now has failed to address the importance of alignment of cooling air or compression jets with the nozzle gas jets. Finally, the prior known apparatus are limited in the range of and control of the spray parameters.
While various apparatus are known, there is yet an apparent need for improved apparatus. These improvements include: better control of the heat source medium, improved material delivery for the separation of functions of material velocity and flow in the heat source and the material meter (spray rate) function, the elimination of a siphon plug mixing device, the ability to cool the flame temperature prior to contact with the polymer feed stock material, the generation of a compression wave for the efficient transfer of a reduced temperature heat source to the polymer feedstock materials, the need for alignment of the cooling mixture air with the nozzle flame jets, the need for the alignment of the compression wave jets with the nozzle flame jets, the ability to spray lower micron electrostatic grade materials (5 to 160 microns) for improved coating homogeneity and smoothness, the ability to apply polymer coatings with little or no need for pre-heating the substrate, the ability to apply polymer materials without consuming the same as a fuel source, the ability to achieve near 100% deposit efficiency of the polymer coating, the ability for non destruction or degradation of the applied polymer coatings by the thermal spray device as the coating is being applied, and the ability to have a greater range and control of the spray parameters.